I actually think that the https://github.com/eclipse/mosquitto broker is a good candidate to port. It is very extensively used, it has tons of test cases that can be used to verify the porting, and its all written in c/c++. The size of the codebase is similar to the https://github.com/eclipse/paho.mqtt.c, and from what I can tell, it has switches to disable external build dependencies, meaning dependencies will be towards a POSIX interface.
The only problem is that you need to port it. Because this has been done before for openssl and paho-mqtt, I don’t expect such an endeavor to be all too dramatic - but then again, it still needs to be done.
cheers